The present invention relates to ultrasound color-Doppler imaging producing in color dynamic information of blood flows within a living body utilizing an ultrasound color-Doppler technique and, in particular, relates to improvement in performance of a filter incorporated to remove clutter components made up of reflections from organs such as the cardiac muscle.
An ultrasound color-Doppler technique utilizes the Doppler effect of ultrasound to non-invasively acquire information concerning dynamics of blood flowing in an object from the outside thereof. Diagnostic ultrasound apparatus capable of executing this technique have advanced remarkably.
One type of diagnostic ultrasound apparatus is known as an apparatus performing a color-Doppler tomography (also referred to as color flow mapping (CFM)). The color-Doppler tomography, based on an MTI (moving target indication) technique which has been used in a field of radar systems, can obtain two-dimensional mapping images of blood velocities in a cross section of an object.
In order to make such velocity mapping images, an ultrasound echo is detected from an object responsively to the transmission of an ultrasound pulse and is converted into an electric signal, before it is branched into a real part signal and an imaginary part signal. Each of the real part and imaginary part signals is phase-detected against a reference signal by an orthogonal phase detector, thereby providing Doppler signals indicative of changes in phase against the reference signal. The real part and imaginary part signals composing a Doppler signal are each digitized by A/D converters and temporarily stored in each buffer memory.
For the CFM mode instructing color flow mapping, an ultrasound pulse is transmitted and received a plurality of times N (for example, 16 times) along the same scanning direction. Digital data necessary for the reconstruction of one CFM image become a three-dimensional volume data having the first to third directions for each of the real part and imaginary part signals and stored in a buffer memory of an MTI filer. The first direction corresponds to the number of each scanning line, the second one the number of pixels existing in each scanning line in its depth direction, and the third one the number of Doppler data obtained by repeating the transmission and reception of an ultrasound pulse.
In this three-dimensional volume data, the same pixel position in a scanned cross section has temporally sequential reception echoes in digital quantities, which are acquired by repeating N-times the transmission of an ultrasound pulse and the reception its echo, phase-detected, and mapped in the third direction. Velocities of changes of a signal represented by data aligned in the third direction indicate values of Doppler shift frequencies corresponding to travel velocities of an object to be imaged.
For the three-dimensional digital data (a group of Doppler signals) stored in the buffer memories of the MTI filter, clutter components are removed at each pixel position every data train aligned in the third direction, on the principle described below.
Received echoes are mixed signals of echo signals reflected from targets which move at velocities of a certain value or more, such as blood cell and echo signals (referred to as clutter components) reflected from tissues such as organs. In terms of signal intensities of echoes, clutter components are larger than blood flows, while in terms of travel velocities of echoes, blood flows are larger than clutter components. Thus a filtering circuit placed in the MTI filter is formed into a high-pass filter and its cut-off frequency is set to a limit at which clutter components are cut off. By this filtering configuration, clutter components are removed from a phase-detected Doppler signal, and echo signals reflected from blood flows are extracted.
The echo signal thus-extracted then undergoes estimation of motion states of blood flows (including blood flow velocities, power, and dispersion) and two-dimensional tomographic images are produced on the estimated information.
Although the conventional MTI filter has been used to remove clutter components in this way, it has not necessarily been satisfactory for removal of clutter components; that is, the conventional MTI filter lacks accuracy and high levels of the removal. This is because the actual organs are in slight motion or happen to move due to various reasons. Clutter components cased by such motion are difficult to clearly be distinguished from the blood flows of slower velocities.
The organs are in motion or tend to move by (1) the beats of the heart (in particular, this becomes a problem when diagnosing blood flows in the heart), motions of surrounding organs depending on shocks from the heart beat, (3) breathing of a patient (body motion), (4) the hand motion of an examining operator, and others.
When setting lower cut-off frequencies of the MTI filter clutter components will not be fully removed, leaving the remaining of the clutter components in the phase-detected signal. In this case, images for motion information of blood flows include the mixed remaining of the clutter components mixed. This results in lowered accuracy of diagnosis of blood flows and there is a problem of misdiagnosis.
In contrast, when setting the cut-off frequency at higher values, most clutter components can be removed steadily, but at the same time, part or most of echo signal components reflected from blood flows are removed. Particularly it is considered that blood flows having lower motion velocities will disappear from the monitor screen. Such images will no longer provide useful information for the diagnosis.
As understood from the above, the removal of clutter components and the extraction of echo signals from blood flows are mutually conflicted on requirements for the cut-off characteristics of signal components. Additionally the blood flow velocities vary with portions to be diagnosed and may vary person to person. In consideration with those conditions, the cut-off frequency of the MTI filter is obliged to be set at compromise values. In consequence, it is difficult for the conventional MTI filter to provide the full removal of clutter components, therefore being compelled to accept such a lower detectability for blood flowing at lower motion velocities.
The present invention has been made in consideration with the above problems posed by the conventionally-used MTI filter. An object of present invention is to provide a diagnostic ultrasound apparatus capable of distinguishing echoes reflected from blood flows of extremely lower velocities from clutter components in a steady and accurate manner, thus producing two-dimensional-mapped blood flow images of higher detectability by steadily removing clutter components caused by organs in motion and by detecting blood flows of extremely lower velocities without fail.